Spatial distribution, sources and health risk assessment of heavy metals in topsoil around oil and natural gas drilling sites, Andhra Pradesh, India

Soils are usually the interface between human activity and environmental components that must be conserved and protected. As a result of rising industrialization and urbanization, activities such as exploration and extraction operations lead to the release of heavy metals into the environment. This study presents distribution of six heavy metals (As, Cr, Cu, Ni, Pb and Zn) in 139 top soil samples collected in and around oil and natural gas drilling sites at a sampling density of 1 site/12 km2. The results indicated the concentration ranged from 0.1 to 16 mg/kg for As, 3–707 mg/kg for Cr, 7–2324 mg/kg for Cu, 14–234 mg/kg for Ni, 9–1664 mg/kg for Pb, and 60–962 mg/kg for Zn. The contamination of soil was estimated on the basis of Index of geo accumulation (Igeo), enrichment factor (Ef), and contamination factor (Cf). Further, spatial distribution pattern maps indicated that the pollution levels for Cu, Cr, Zn, and Ni were higher around drilling sites of the study area relative to other regions. Using exposure factors for the local population and references from the USEPA’s integrated database, potential ecological risk indices (PERI) and health risk assessments were made. The hazard index (HI) values of Pb (in adults) and Cr, Pb (in children) exceeded the recommended limit of HI = 1, indicating the non-carcinogenic risks. Total carcinogenic risk (TCR) calculations revealed Cr (in adults) and As, Cr (in children) levels in soils exceeded the threshold value of 1.0E − 04, indicating significant carcinogenic risk due to high metal concentrations in the study area. These results may assist in determining the soil’s present state and its effect due to extraction strategies used during drilling process and initiate few remedial techniques, particularly for proper management strategies in farming activities to decrease point and non-point source of contamination.

Gondwana sedimentary sequences formed in the Godavari Graben. The sedimentary rocks show a NW-SE trend and is found to lie between the Dharwar Craton and the Bastar Craton almost perpendicular to EGMB. The rifting and associated tectonics during Gondwana from Permian to Carboniferous resulted in the deposition of thick sedimentary packages representing divergent sedimentary environments from Glacio-lacustrine to arid www.nature.com/scientificreports/ and fluvial environments and depositional history which display multiple sets of faults signifying considerably long deformational history. The sedimentary package is represented by sandstone and shale of Gondwana while the tertiaries consists of alluviums including black clay with sand, black/brown silty clay etc. The sedimentary package and the alluviums act as sink for organic and inorganic metal/metalloids in these fluvial deposits contributed to the heavy metal budget in the region forming one of the prominent geogenic source 48 .
Sampling and preparation. A total of 139 soil samples were collected from the entire study area at a sampling density of 1 sample/12 km 2 grid. Soil sampling was done with a shovel from the surface, at a depth of 20 cm, to investigate the anthropogenic source of pollutants, as industrial pollutants often pollute the top layer of the soil (0-40 cm). Figure 1 shows a study area map with soil sample locations. After every sample collection the shovel was washed with Milli-Q water to avoid contamination. Soil samples were stored in self-locking polythene bags and dried in the open sun and finely powdered using an agate mortar to a 250-mesh size (US Standard) for chemical analysis. The global positioning system was used to record the geographic location of each sampling location. Finely powdered representative samples were chemically dissolved in supra pure HF and HNO 3 (7:3 ratio) in HDPE vials (Savillex®, USA) taking 0.05 g and completing the digestion by heating the content at 70 °C for about 48 h. Rh 49 was used as an internal standard before making the final volume to 250 ml. The duplicates and blank were also prepared following the same procedure. Clear solutions were obtained for all the samples which were analyzed for trace elements (As, Cr, Cu, Ni, Pb and Zn) and rare earth elements (La to Lu) using a high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) (Make: Nu Instruments, UK; Model: ATTOM®) at CSIR-National Geophysical Research Institute, Hyderabad.
Quality assurance and quality control. Geochemical certified reference materials SO-1, SO-2, SO-3, SO-4 from CANMET, Canada, and JSd-1, JSd-2, JSd-3 from GSJ, Japan were utilized for calibration and checking accuracy/precision of the analysis. The analysis was carried out at moderate resolution (R = 300) with peak scan mode by which the analytes of interest is precisely determined sequentially 50 . In most of the cases, the precision and accuracy were found to me within acceptable limits varying from 2 to 5% RSD (Table 1).

Geographical information system. A Geographical information system (GIS) facilitates the collection
and modification of a vast array of geographical data and presents the spatial distribution of all parameters 51 . Geographic information systems (GIS) enable the modification of all parameters affecting soil quality. This provides individuals and policymakers with a comprehensive understanding of the situation. Using the inverse distance-weighted interpolation method of the ArcGIS-10.7 program (CSIR-NGRI, Hyderabad, India), a map of the geographic distribution of all parameters was made. www.nature.com/scientificreports/

Analysis methods
Multivariate statistical methods. The analytical data were processed using certain statistical techniques to examine the distribution and correlation of the parameters under study. Statistical SPSS 10.0 was used to compute and evaluate the heavy metal data using multivariate statistics like Principal component analysis (PCA) 52,53 . Principal components provide information on the most meaningful parameters which describe whole data set affording data reduction with minimum loss of original information. PCA is a powerful technique for pattern recognition that attempts to explain the variance of a large set of inter-correlated variables and transforming into a smaller set of independent (uncorrelated) variables (principal components). These components further reduce the contribution of less significant variables obtained from PCA and the new group of variables known as varifactors are extracted through rotating the axis. On the dataset, PCA was achieved using varimax normalized rotation.

Pollution indices.
For measuring heavy metal pollution in soil, a variety of single-and Integrated Indices approaches are available. These factors frequently make sure to give a qualification of pollution as opposed to quantification of contamination as a result of the many different factors that might influence their utility. The most essential difficulty is that until historical data is available, it is sometimes quite difficult to determine what the actual composition of the soil/sediment was in terms of the components of relevance. Also, the issue is the demand for an analog of non-contaminated soil/sediment if there is no historical data. This necessitates taking a sample from somewhere other than the polluted location. This poses the question of how to account for heavy metal inputs from sedimentary and lithogenic sources. Calculating environmental quality indices (soil/sediment quality indices) is the basis of quantitative techniques. These techniques are regarded as an excellent tool for modifying environmental data into information that non-specialists can understand 54 . Soil Quality Classification for Pollution Indices is shown in Table 2.
Index of geoaccumulation (I geo ). The Index of Geoaccumulation offers an evaluation of pollution by assessing modern and pre-industrial concentrations in bottom sediments. Additionally, this can be utilized in the evaluation of soil pollution 55 . The following equation was used to calculate it: where Cn is the element's observed concentration in the samples, and Bn is either from the literature or directly measured in texturally similar uncontaminated soils. The constant 1.5 allows us to investigate regular variabilities in the concentration of a specific material and minor human impacts in the environment. In this investigation, we used a modified calculation based on Loska Equation 56 , where Cn signified the content of a certain element in the soil analyzed, and Bn represented the content of elements in the earth's crust 57 .
Enrichment factor (E f ). Loska [59][60][61] . The reference element used in this study was Ni. Nickel was chosen as the standard element because there is a lot of it in the Earth's crust, but the amount of it in soils varies depending on their matrices. As a reference environment that was comparable to I geo , the average elemental concentration in the Earth's crust was used.
(1) I geo = Log (Cn/1.5 B n ) Contamination factor (C f ). The modified method for calculating the contamination factor was used in this study 62 . The contamination factor is calculated using the formula below.
Co represents the average metal content of at least five sample locations, and Cn represents the elemental concentration in the earth's crust. Hakanson classified contamination factor into four groups.
Potential ecological hazard index. Ecological Risk Assessment (ERA) methodology to evaluate the potential ecological risks posed by metals in soil and sediment 62 . In order to assess index-level risk, an ecological risk assessment investigates the toxicological, ecological, and environmental impacts of soil pollution and metals [63][64][65] . The ecological risk is calculated by using Eq. (4) where C i is the sample metal concentration and C 0 is the background metal concentration 56 . T i r represents the toxic response factor of a specific metal. The toxic response factors (TRFs) for Cu (5), Cr (2), Pb (5), Zn (1), Ni (5), and As (10) were derived from the literature 62,66,67 . The risk (RI) values can be calculated by Eq. (5) On the basis of assessment, the Er values < 40, RI < 150 shows low ecological risk, 40 ≤ Er < 80, 150 ≤ RI < 300 shows moderate ecological risk, 80 ≤ Er < 160; 300 ≤ RI < 600 indicates considerable ecological risk, 160 ≤ Er < 320; RI ≥ 600 shows high ecological risk and Er ≥ 320 considered very high ecological risk.
Human health risk assessment.. Exposure assessment. The health risk assessment is a reliable technique for estimating the harm presented to humans by contamination. Humans are often exposed to heavy metals in soils via three paths: (a) direct oral ingestion of sand grains; (b) dermal absorption of heavy metals in soil particles attached to exposed skin; and (c) mouth and nose inhaling of soil particles [68][69][70][71][72][73][74][75][76][77][78][79][80] . Calculations are performed using the following equations to determine the chronic daily dose (CDD: mg/kg/day) of potentially hazardous heavy metals absorbed via the three various exposure to non-carcinogenic heavy metals.
Soil ingestion was selected as the most preferred approach for humans exposed to environmentally hazardous components. Calculations and estimates of the chronic risk to the population of both adults and children (male and female) from soil intake were performed. In general, children have more direct contact with soil than adults, ingest more soil unintentionally, and are exposed to soil more often. In addition, it is believed that contaminants have a stronger influence on underweight children. Table 3 shows all of the exposure variables and values used to determine intake values and risk.
(2) E f = C n sample / C ref sample / B n background / B ref background (3) www.nature.com/scientificreports/ Non-carcinogenic risk assessment. The hazard quotient (HQ) is a commonly used to measure for assessing non-carcinogenic risk. It is determined by dividing the chronic daily dose (CDD) by the reference dose (RfD) for a certain chemical 38,45 . The hazard index (HI) is the total of the HQ and means of the overall risk of noncarcinogenic components for a particular element based on three categories of risks. Following formulae determine HQ and HI.
As per USEPA (1989), RfD is the guideline dose for each heavy metal as shown in Table 4. If the HI number is less than one, there is no threat of non-carcinogenic consequences occurring; if the HI value is higher than one, there is a possibility of potential non-carcinogenic impacts on health.
Carcinogenic risk assessment. Carcinogenic risk is the likelihood that a human may acquire any sort of cancer over the course of their life owing to exposure to carcinogenic risks 38,74,75,81 . The following formula is used to evaluate the lifetime carcinogenic health hazards associated with a particular heavy metal 45,76 .
where CR denotes the carcinogenic risk, TCR represents the total carcinogenic risk, and SF indicates the slope factor (mg/kg/day) as shown in Table 4. The US Environmental Protection Agency (1989) suggests that a CR and TCR value of less than 1 × 10 −6 should be considered inconsequential, whereas a CR and TCR value that exceeds 1 × 10 −6 is considered to be hazardous to human health.

Results and discussion
Descriptive statistical summary of heavy metals in soils. The obtained elemental concentrations with descriptive statistical variables for the six heavy metals (As, Cr, Cu, Ni, Pb, and Zn) employed in this study together with background values 57 and Canadian soil quality guidelines values 82 are shown in Table 5. In the soil concentrations for individual heavy metals, ranges showed 0.10 to 15.9 mg/kg for As, 2.94 to 707.1 mg/kg for Cr, 6.86 to 2324 mg/kg for Cu, 14.27 to 234.0 mg/kg for Ni, 9.03 to 1664.5 mg/kg for Pb, and 60.7 to 961.8 mg/kg for Zn, with average concentrations of 3.2, 203.3, 253.3, 73.1, 68.6, and 229.6 (mg/kg) respectively. For evaluating contamination in the study area, background soil concentrations of the examined components are essential. The mean concentrations of As, Cr, Cu, Ni, Pb, and Zn largely exceeded Taylor and McLennan 1995, but when compared to Canadian soil quality guidelines Cr, Cu, and Ni metals exceeded guideline values, and the remaining metals were within the limits, indicating that surface soils are greatly polluted by these heavy metals. Produced water is usually formation water from oil and gas extraction. It may also contain dissolved inorganic salts, hydrocarbons, minerals, trace metals, naturally occurring radioactive substances, production chemicals, and dissolved gases [83][84][85][86] . This phenomenon is attributable to the extensive use of chemical substances containing high concentrations of heavy metals discharged into the environment due to petroleum drilling and extraction. The effects of petroleum exploration and production operations on the heavy metal contents of soil and groundwater in the Niger Delta 87 and also, one of the investigations revealed that the soil in close proximity to sites where oil and  (Table 6).
Spatial distribution maps. Heavy metal concentration maps are an extremely useful tool for delineating zones with significant metal contents in the study area. The spatial distribution patterns of As, Cr, Cu, Ni, Pb, and Zn (Fig. 3) in the study area's top soils were prepared. Samples taken near drilling sites revealed the highest amounts of all harmful elements. Diverse concentrations of the six constituent elements were observed across different drilling regions, suggesting a degree of dispersion of hazardous elements due to various extraction processes and soil degradation mechanisms. The distribution of As towards west side of the study area indicates the probable source would be from agriculture activity due to usage of pesticides. Cu, Pb, Ni and Cr are observed to be exceeding the reference values given by Canadian guideline and Taylor McLennan values. This is mainly observed nearby drilling sites, with major source from back water containing various inorganic compounds and thereby getting deposited on to the surface. Overall the distribution of heavy metals is concentrated towards south-west of the study area due to major drilling activity, when compared to north-east where the drilling activity is minimal.

Index of geo-accumulation (I geo ).
The concentrations of the six heavy metals in the soils were calculated (Table 7) using the Igeo index ( Fig. 4) The study area exhibited a wide range of contamination levels, ranging from pristine to highly contaminated. The I geo value for As ranged from − 2.83 to 1.2 and was greater than 0 for 82% of the soil samples, classifying them as slightly to moderately polluted. Enrichment factor (EF). The enrichment factor was used to determine soil heavy metal concentrations ( Fig. 5). Table 7 shows that the EF values for Cr, and Zn in soils varied from 0.1 to 4.9, and 0.2 to 4.9, with 25.2%, and 5% respectively, of soil samples classified as having indicating deficiency to minor enrichment. However, the EF of As, Cu and Pb ranged from 0.0 to 8.1, 0.3 to 28.7 and 0.0 to 25.7 with 10.1%, 31% and 11% respectively, of soil samples falling into moderate to significant enrichment. The maximum levels documented for arsenic, copper, and lead were obtained from a solitary low-capacity produced water source from a gas and condensate field. This study examines the environmental effects of the Norwegian offshore petroleum industry's discharge of produced water and drilling waste 86 .

Contamination factor (CF).
Using the contamination factor (Fig. 6), the levels of the six different heavy metals calculated in the soil were determined. Based on calculations in Table 7, the mean CF values observed for As and Cu in soils were 2.1 and 0.9, respectively. The coefficient of variation (CF) values for the elements above  . This factor can be attributed to anthropogenic influence of these heavy metals, probably as a result of back water or produced water from the drilling rigs, which contain various inorganic chemicals with these metals as by-products. The high values of Cu and Pb are observed in the old and active drilling sites at Mandapeta and Bantumilli of the study area. Further, the Pb being one of the major chemicals among others like water, sand, salt, citric acid this component can be attributed to anthropogenic. The chemicals that gas companies use include water. Factor PC2 showed Cr and Ni as significant loadings which explained 52.85% of the overall variation. The positive loadings of Cr (r = 0.779) and Ni (r = 0.820) indicate the source to be attributed as anthropogenic/geogenic. This is observed in Nagendrapuram and Arjunudupalem drilling sites of the study area. Further, the source of Cr could be a contaminant in the chemical stew used in fracking fluids, but it also occurs naturally in the ground, so the possibility exists that it might picked up there by fracking fluids pumped into deep gas wells under enormous pressure and then discarded after the drilling is complete. Whereas, the source of Ni would be from the chemicals used in fracking fluids where Nickle sulphate being one of the inorganic compound. PC3 represented 28.68% of the total variance and loadings on Zn with positive loading (r = 0.739), observed in Prathallamaraka drilling site.
Assessment of potential ecological risk. The potential ecological risk index (PERI) values of six different heavy metals discovered (Table 9) in the area of the study's topsoil were calculated. All of the elements As, Cr, Cu, Ni, Pb, and Zn (Fig. 7) exhibited mean values of Efi that were lower than 40, with mean values in the order As (14.2) > Ni (0.9) > Pb (0.8) > Cr (0.3) > Cu (0.1) > Zn (0.04) representing that these metals posed a low potential ecological risk. It is important to note, however, that several of the sampling sites revealed a potential ecological risk that ranged from moderate especially for arsenic, the maximum value was 0.0 to 70.9. In contrast As is much complex than those of the other heavy metals, 15% of all samples for arsenic demonstrated a moderate www.nature.com/scientificreports/ potential ecological risk level. Similar to this, many authors in their studies proposed that using arsenic-enriched fertilizers and pesticides or irrigating crops with arsenic-contaminated water could lead to higher levels of arsenic in soil 49,103 . Few mining areas in China observed in PERI results that As metal showed moderate potential ecological risk and suggested that As is likely derived from other sources such as mining and related industrial activities 67 . The results of RI ranged in between 0.3 and 72.3, with a mean of 16.6, indicating that six heavy metals of soils of the study area were in low risk category (Table 9).
Non-carcinogenic health risks. Non-carcinogenic health risk assessments in surface soils were performed on As, Cr, Cu, Ni, Pb, and Zn using primary exposures: ingestion (CDD ingestion), dermal (CDD dermal), and inhalation (CDD inhalation) for both adults and children (male and female). The HQ of adults and children due to three exposure routes decreased as follows: Ingestion > Dermal > Inhalation. Furthermore, demonstrates that the non-carcinogenic health consequences of exposure to heavy metals for male adults (Fig. 8)   www.nature.com/scientificreports/ 1.0E − 02 to 1.9E + 00 for male adults, indicating that Pb had some negative impacts on male Adults in the study region. Table 10 presents the non-carcinogenic health effects of heavy metal exposure through ingestion for adult females (Fig. 9) These results indicate that Cr and Pb in the study region may adversely affect female adults. For both male and female adults in the study region, the HI was found to be in the following order: Pb > Cr > As > Cu > Ni > Zn. It appears there is no considerable danger to male adult health from the non-carcinogenic effects of As, Cr, Cu, Ni, and Zn and to female adult health from the non-carcinogenic effects of As, Cu, Ni, and Zn in the study area. Table 10 further indicates that the non-carcinogenic health effects of heavy metal exposure for male children (Fig. 10)   www.nature.com/scientificreports/ reveals that the hazard quotient (HQ) of chromium (Cr) and lead (Pb) was significantly higher in comparison to other heavy metals because of their significantly raised levels in the soil of the investigated region. The study observations reveal that the estimated hazard index (HI) values for Cr and Pb ranged from 7.4E − 03 to 1.8E + 00 and 6.6E − 02 to 1.2E + 01, respectively, for male children. These results suggest that Cr and Pb may adversely affect male adults in the study area. Also, it has been demonstrated that ingesting heavy metals can result in non-carcinogenic health effects for female children (Fig. 11) For both male and female children in the study region, the HI was found to be in the following order: Cr > Pb > As > Cu > Ni > Zn. It appears there is no considerable risk to male and female children health from the non-carcinogenic effects of As, Cu, Ni, and Zn in the study area because the HI values are all lower than the safe limit (HI = < 1) for all sampling points. The study found that the amounts of HQ exposed to adults and children via skin contact and inhalation were substantially lower than those exposed to HQ through ingestion, www.nature.com/scientificreports/ indicating that these pathways pose no health concern. This indicates that the greater concentration of soils in the study area indicates that the hazard quotient (HQ) of Cr and Pb is higher than for other heavy metals. The non-carcinogenic risk level distributions of the study area were mapped, and which Cr and Pb exhibited the most severe scenarios were observed. The non-carcinogenic risk values of both adults and children were used in the present study. It indicates that the non-carcinogenic risk values in provinces surrounded by drilling rigs are comparatively higher than in various other areas.
Carcinogenic health risks. Individuals are growing substantially the risk of cancer through being exposed to toxins including As, Cr, and Pb. The USEPA considers carcinogenic risks (CR) and total carcinogenic risks (TCR) levels below 1.0E − 06 to be minimal, whereas values exceeding 1.0E − 04 can become dangerous to human health (USEPA 1989(USEPA , 2002. Table 11 depicts the results of 3 different treatment paths for As, Cr, and Pb for adults and children (male and female) (Figs. 12 and 13). However, the TCR of Cr for adults in both male and female ranged from 7.1E − 03 to 1.7E + 00 and 7.3E − 03 to 1.8E + 00 with a mean of 4.9E − 01 and 5.1E − 01 respectively; its acceptable threshold value of 1.0E − 04 was exceeded by about 97.8 and 98.5% of the sample locations, respectively (USEPA 2002). Further, TCR values of As and Cr for children in males and females varied from 1.1E − 04 to 1.2E + 00, 3.7E − 02 to 8.8E + 00, and 1.1E − 04 to 1.2E + 00, 3.6E − 02 to 8.6E + 00 with a mean of 2.5E − 01, 2.5E + 00 and 2.5E − 01, 2.5E + 00 respectively; About all of the sampled locations exceeded the threshold value, which is considered acceptable. The carcinogenic health hazards to adults and children from exposure to As and Cr have typically greater potential carcinogenic health risks when compared to other heavy metals, and the Pb risks posed to adults and children (male and female) from exposure were considerably less in the study area. Many authors have noticed that the surface soils surrounding an industrial place in India proved more carcinogenic to children than adults 104 . Many other areas have comparable results 68,[105][106][107][108][109] . Few studies show that emerging soil pollution increased carcinogenic and non-carcinogenic health risks in every area 110 . It was also observed in a study that the levels of As and Cr in the soil of an example county in Shanxi Province, China exceeded the acceptable limit of 1.0E − 04 111 . This result suggests that children in the area may be exposed to carcinogenic risks through ingestion, dermal contact, and inhalation pathways. According to results, the heavy metals discharged from the drilling operations have significantly contaminated the soils close to the oil and gas drilling locations, according to health and environmental risk assessments. In addition, soil www.nature.com/scientificreports/ contamination with heavy metals remains a significant source of carcinogenic and non-carcinogenic hazards for the general population, with particular emphasis on vulnerable groups such as children and individuals residing in areas with the highest pollution levels.
Potential health hazards. Various short-and long-term toxicological impacts of heavy metals have been observed to impact diverse internal organs. Heavy metals may cause gastrointestinal dysfunction, renal dysfunction, neurological diseases, skin lesions, vascular damage, immune system malfunction, birth abnormalities, and cancer. Cumulative effects may result from contemporaneous exposure to more than one metal [112][113][114][115][116] . Arsenic, considered a hazardous heavy metal, is a significant contributor to the endangerment of human health. The substance As has a lengthy historical record of use as a metalloid element and a therapeutic agent. The potential adverse outcomes encompass hyperpigmentation, cutaneous abnormalities, and elevated susceptibility to neoplastic growth. Cr has the potential to induce a range of ailments via its process of bioaccumulation within the human body. This includes skin, kidney, nervous system, and gastrointestinal diseases, as well as cancers of the lungs, throat, bladder, kidneys, ovaries, bones, and thyroid 117,118 . Pb may cause neurological, respiratory, urinary, and cardiovascular issues. Immune modulation, oxidative damage, and inflammation cause these consequences. www.nature.com/scientificreports/ Exposure to lead (Pb) was found to cause changes in the physiological functions of the human body and is linked to the development of numerous illnesses 119-121 .

Conclusion
The study described the preliminary distribution of heavy metals in soils and their probable source distribution around oil and natural gas drilling sites and presented obvious disparities and varied correlations among selected metal contents (As, Cr, Cu, Ni, Pb and Zn) which were higher than their background values in the study area. According to the results, the higher concentration of heavy metals could be attributed to both natural and anthropogenic source of contamination, majority would be anthropogenic input from backwater waste generated during drilling. Apart from oil drilling sites, the other probable source of anthropogenic contamination due to heavy metals can be attributed as well due to agriculture which are active in the study area. The multivariate revealed significant anthropogenic pollution especially by As, Cr, Cu and Pb in soils of study area. As a result of the index of geoaccumulation, enrichment factor and contamination factor very high As, Pb and Cr concentrations were found in the soil and may be mix with groundwater by leaching and posing serious threat to human health. These risk indices when applied to the soils of the study area revealed high risk of Figure 9. Spatial distribution of non-carcinogenic health risk due to heavy metals in Adult (female) (maps were generated with software ArcGIS 10.7 http:// www. esri. com). www.nature.com/scientificreports/ contamination by As, Pb and Cr and the order of ecological risk index was As > Pb > Ni > Cr > Cu > Zn. The risk of chronic disease occurrence is generally low in adults and medium to high in children. Some of the sampling points indicated high potential risk of As and Pb as these had medium to high chronic as well as carcinogenic risk. The study area shows a potentially greater risk due to anthropogenic contamination. Assessment of health risk from toxic elements like As, Pb and Cr contamination in soils have been mapped in the form of distribution maps which will highlight the potential risk zone. www.nature.com/scientificreports/

Recommendations
The recommendations in the study area would be to monitor the soil parameters by applying basic quality testing and to monitor the activities in the drilling sites. To further contain the backwater chemicals generated which are harmful to human health, biological treatment has been an effective but infrequently used for oil and gas produced water. Few biological treatments of produced water include information for fixed-film treatment, membrane bioreactors, wetlands and ponds, activated sludge treatment, anaerobic treatment, and bio-electrochemical treatment, and biopreparation for the bioremediation of oil-contaminated soils. Further, suggest suitable remedial measures to avert contamination due to heavy metals in near future. Figure 10. Spatial distribution of non-carcinogenic health risk due to heavy metals in. Children (male) (maps were generated with software ArcGIS 10.7 http:// www. esri. com). www.nature.com/scientificreports/ Figure 11. Spatial distribution of non-carcinogenic health risk due to heavy metals in Children (female) (maps were generated with software ArcGIS 10.7 http:// www. esri. com). www.nature.com/scientificreports/ www.nature.com/scientificreports/